1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to an adaptive driving apparatus for a liquid crystal display that prevents a residual direct current component from flowing in a liquid crystal.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) with an active matrix driving system uses thin film transistors (TFT's) as switching devices to display a natural moving picture. Because LCDs can be placed into a device smaller in size than existing cathode-ray tubes, it has been widely used as a monitor for personal or notebook computers as well as for office automation equipment such as copy machines, etc. and portable equipment such as cellular phones and pagers, etc.
The active matrix LCD displays a picture corresponding to video signals, such as television signals, on a picture element matrix, or pixel matrix, having liquid crystal cells arranged at crossings of gate lines and data lines. A thin film transistor is provided at each intersection between the gate lines and the data lines to thereby switch a data signal to be transmitted into the liquid crystal cell in response to a scanning signal (or gate pulse) from the gate line.
An LCD may be classified into either an NTSC signal system or a PAL signal system in accordance with a television signal system with which the device is to be used.
Generally, if an NTSC signal (i.e., 525 vertical lines) is inputted, then a horizontal resolution of the LCD is expressed in accordance with the number of sampled data while the vertical resolution thereof is expressed by a 234 line de-interlace scheme. On the other hand, if a PAL signal (i.e., 625 vertical lines) is inputted, then a horizontal resolution of the LCD is expressed in accordance with the number of sampled data while a vertical resolution thereof is expressed by a processing system similar to the NTSC signal scheme in which one line is removed for each six vertical lines to result in 521 lines.
Referring to FIG. 1 and FIG. 2, a related art LCD driving apparatus includes a liquid crystal display panel 30 having liquid crystal cells arranged in a matrix type, a gate driver 34 for driving gate lines GL of the liquid crystal display panel 30, a data driver 32 for driving data lines DL of the liquid crystal display panel 30, an image signal processor 10 for receiving an NTSC television signal and applying television complex signal, divided into RGB data signals R, G and B, to the data driver and to output a complex synchronizing signal Csync, and a timing controller 20 for receiving the complex synchronizing signal Csync from the image signal processor 10 to output a horizontal synchronizing signal Hsync and a vertical synchronizing signal Vsync and for generating a polarity inversion signal FRP that is applied it to the image signal processor 10, thereby controlling the data driver 32 and the gate driver 34.
The liquid crystal display panel 30 includes liquid crystal cells arranged in a matrix, and thin film transistors TFT provided at intersections between the gate lines GL and the data lines DL to be connected to the liquid crystal cells.
The thin film transistor TFT is turned on when a scanning signal, that is, a gate high voltage VGH from the gate line GL, is applied. This applies a pixel signal from the data line DL to the liquid crystal cell. The thin film transistor TFT is turned off when a gate low voltage VGL is applied from the gate line GL, to thereby maintain a pixel signal charged in the liquid crystal cell.
The liquid crystal cell can be equivalently expressed as a liquid crystal capacitor Clc, and includes a pixel electrode connected to a common electrode and the thin film transistor TFT that are opposite each other and having a liquid crystal therebetween. Further, the liquid crystal cell includes a storage capacitor Cst for maintaining the charged pixel signal until the next pixel is charged. This storage capacitor Cst is provided between a pre-stage gate line and the pixel electrode. Such a liquid crystal cell varies an alignment state of the liquid crystal having a dielectric anisotropy in response to the pixel signal charged via the thin film transistor TFT to control a light transmittance, thereby implementing a gray scale level.
The image signal processor 10 applies a gamma treatment of image signals (NTSC) supplied from the exterior thereof in consideration of a characteristic of the liquid crystal display panel 30, and converts polarities of the image signals (NTSC) using the polarity inversion signal FRP from the timing controller 20 for the purpose of prolonging a life of the liquid crystal, thereby generating RGB data. Further, the image signal processor 10 separates the complex synchronizing signal Csync from the image signals (NTSC) and applies it to the timing controller 20, and applies the RGB data to the data driver 32.
The timing controller 20 includes a frequency divider (not shown) for outputting a frequency-dividing signal having the same period as the complex synchronizing signal Csync and various clocks, and synchronizes the complex synchronizing signal Csync with the frequency-dividing signal with the aid of the phase locked loop PLL. The frequency-dividing signal is synchronized with a center portion of the width of the complex synchronizing signal Csync. The timing controller 20 generates a horizontal synchronizing signal Hsync inverted with respect to the complex synchronizing signal Csync using various clocks from the frequency divider. Further, the timing controller 20 generates data control signals SSP, SSC and SOE for controlling the timing of the data driver 32, and generates gate control signals GSP, GSC and GOE for controlling the timing of the gate driver 34 in order to apply it to the gate driver 34.
Moreover, the timing controller 20 includes a polarity inversion circuit for converting the polarities of the image signals (NTSC). This polarity inversion circuit applies a polarity inversion signal FRP for inverting the image signals (NTSC) to the image signal processor 10 during each desired period, such as, for each one field period or for each one horizontal period, in order to prevent the deterioration of the liquid crystal caused by residual direct current components applied to the liquid crystal.
The gate driver 34 sequentially applies the gate high voltage VGH to the gate lines GL in response to the gate control signals GSP, GSC and GOE from the timing controller 20. Thus, the gate driver 34 drives the thin film transistors TFT connected to the gate lines GL for each gate line.
More specifically, the gate driver 34 shifts a gate start pulse GSP in response to a gate shift pulse GSC to generate a shift pulse. Further, the gate driver 34 applies the gate high voltage VGH to the corresponding gate line GL every horizontal period H1, H2, . . . in response to the shift pulse. In this case, the gate driver 34 applies the gate high voltage VGH only in an enable period in response to a gate output enable signal GOE. On the other hand, the gate driver 34 applies the gate low voltage VGL in the remaining period when the gate high voltage VGH is not applied to the gate lines GL.
The data driver 32 applies pixel data signals for each horizontal line to the data lines DL every horizontal period 1H, 2H, . . . in response to data control signals SSP, SSC and SOE from the timing controller 20. Particularly, the data driver 32 applies RGB data from the image signal processor 10 to the liquid crystal display panel 30.
More specifically, the data driver 32 shifts a source start pulse SSP in response to a source shift clock SSC to generate a sampling signal. Then, the data driver 32 sequentially inputs analog RGB data for each unit in response to the sampling signal to latch them. Further, the data driver 32 applies the latched analog data for one line to the data lines DL.
The related art LCD driving apparatus and method controls polarities of image signals (NTSC) applied to the liquid crystal display panel 30 using the polarity inversion signal FRP applied from the timing controller 20 to the image signal processor 10, thereby preventing residual current components from flowing in the liquid crystal and thus preventing a deterioration of the liquid crystal.
Meanwhile, the conventional LCD driving apparatus and method supplies and displays the same data on at least two horizontal lines during one horizontal period when image signals A adopting the NTSC system are displayed, thereby being enlarged into the entire field of the liquid crystal display panel 30 as shown in FIG. 2. When RGB data are displayed enlarged in the vertical direction of the liquid crystal display panel 30, a zero-level voltage or a desired level of direct current voltage is applied to the liquid crystal for a long time. Therefore, if a direct current voltage is left at the liquid crystal for a long time, then the liquid crystal molecules deteriorate.